1. Field of the Invention
The present invention relates to a transfer material carrying member and an image forming apparatus. More particularly, it is concerned with a transfer material carrying member used when a toner image formed by electrophotography or electrostatic recording is transferred to a transfer material, and an image forming apparatus having such a transfer material carrying member. The image forming apparatus herein includes black and white, monochromatic or full-color electrophotographic copying machines, printers and other various recording apparatus.
2. Related Background Art
Various members are known as transfer material carrying members used when an image on an image bearing member is transferred to a transfer material. For example, in an electrophotographic apparatus having an image forming means comprised of the steps of charging, imagewise exposure, toner development, transfer and cleaning, a means for transferring a toner image on a photosensitive member to a transfer material as exemplified by paper may include a transfer drum and a transfer device as shown in FIGS. 1 and 2, respectively.
A transfer drum 10 comprises a support comprised of cylinders 12 and 13 provided on its both ends and a connecting part 14 that connects these cylinders. A transfer material carrying member 11 is so provided on this support as to extend through an opening made in its peripheral wall. The connecting part 14 is provided with a transfer material gripper 15 that holds a transfer material fed from a paper feeder. A transfer charge assembly 21, and an internal charge-eliminating discharge assembly 23 and external charge-eliminating discharge assemblies 22 and 24 that constitute a charge-eliminating means are also provided inside and outside the transfer drum 10.
In the transfer step, various mechanical and electrical external forces are applied to the transfer material carrying member 11 during transport of transfer materials, transfer charging, charge elimination, cleaning and so forth, and hence the transfer material carrying member is required to endure to such external forces, that is, to have mechanical strength, wear resistance and electrical durability as well as an excellent surface lubricity to a cleaning member or the like.
Films of resins such as Teflon, polyester, polyvinylidene fluoride, triacetate and polycarbonate have been hitherto used as transfer material carrying members. When, however, these resin films are used as transfer material carrying members, release discharge occurs when a transfer material is released from a photosensitive drum immediately after transfer, so that the transfer material is statically charged because of this release discharge. The charges thus produced can not escape anywhere and are held by the transfer material and the transfer material carrying member, often bringing about disorder of toner images on the transfer material or non-uniform charging for the subsequent transfer. On such occasions, it has been necessary to strictly set transfer conditions, e.g., to precisely control transfer current values or to eliminate residual charges on the transfer material carrying member by means of reverse charging or AC charging.
To solve such problems, Japanese Patent Application Laid-open No. 60-10625 discloses a method in which carbon black is dispersed in a resin film used as a transfer material carrying member, to arbitrarily control volume resistance of the resin film.
Such a carbon-dispersed film, however, tends to have a relatively low transparency, and hence use of this film as a transfer material carrying member in an image forming apparatus may give a limitation on the position at which an optical sensor is provided. Accordingly, studies have been made on transfer material carrying members having a higher light transmittance.
In recent years, it has become popular to use in a developer what is called small-diameter toner particles, having particle diameters of 10 .mu.m or less and an average particle diameter of about 8 .mu.m, in order to make a latent image highly detailed so that images can have a higher image quality, and in order to improve reproduction of such a latent image. Hence, the toner particles tend to pick up a very slight uneven potential produced on a transfer material carrying member in the transfer step. Thus, studies have been also made on transfer material carrying members that do not tend to cause charge nonuniformity.